Negative working diazo sulfonate foils



- 2,854,338 Patented Sep 9 1958 2,854,338 NEGATIVE WORKING DIAZO SULFONATE FOILS Clifford E. Herrick, .lr., Chenango Forks, and Ann H. Balk, Woodstock, N. Y., assignors to General Aniline & Film Corporation, New York, N. Y.,.a corporation of Delaware No Drawing. Application March 18, 1955 Serial No; 495,358

9 Claims. (Cl. 96-75) The present invention pertains to a negative working diazo sulfonate foil in which the diazo sulfonate is imbibed to a layer of a mixture of a maleic anhydridevinyl compound copolymer and a hydrophobic resin compatible therewith, said copolymer having been treated to produce water sensitivity for application of the diazo sulfonate and the utilization of such a foil'in the preparation of colored copies of good actinic opacity.

It has been recognized that diazotype materials containing diazo sulfonate, unlike other diazotype materials, have the peculiarity when exposed under a pattern and developed by means of an alkali of yielding an image opposite in character to that in the pattern. Thus, if the image in the pattern is negative, that produced in the diazotype material will be positive, and vice versa. Although this fact has been knownfor years, no satisfactory negative working diazo sulfonate process has ever been commercially developed. In this connection, reference is made to the article by Van der Grinten entitled, The Diazotype Printing Process, which appears in the Photographic Journal, vol. 92B, 1952, pages 43 et seq., in which Van der Grinten, in paragraph 3 of page 43 states:

Neither the negative diazotype process" of the type of Dr. Wests process, the earliest diazotype process of all, nor that of the type of the Peer process which is based on the use of diazosulphonates, have; been commercialized despite the interest the latter had raised for copying from continuous tone negatives, followed by re-copying on positive diazotype materials and for use in making multicolour pictures.

It is believed to be self-evident that the possibilities inherent in the diazo sulfonate procedure have much to recommend it. The very fact that the art has failed to realize these potentialities under-lines the problems which the commercial use thereof entails. These, it may be said, center mainly around the inability ofprior operators to produce a carrier for the diazo sulfonate which would meet commercial standards.

The negative working diazotype materials were initially made by utilizing, as the carrier for the sensitizer, a surface-saponified layer of cellulose acetate. More recently, there was employed as such carrier a hardened gelatin coated foil or paper. The utilization of these water sensitive materials, however, presents serious obstacles. Thus, the surface-saponification of cellulose acetate is a difficult technical procedure for the reason that it involves the repeated application of a saponifying solution together with adequate neutralizing and washing operation to remove excess caustic. The saponifying solution generally consists of an alkaline water-organic solvent mixture which must be carefully controlled with respect to theconcentration of all components during use. This necessarily means that'- saponification' is not only difficult but expensive.

Similarly, the application of gelatin to a transparent base, such as cellulose acetate, isan involved procedure requiring complicated subbing techniques, elaborate coating and driving facilities and, in general, all the equipment and technical knowledge required in preparing photographic filmbase. Furthermore, washed gelatinlayers require long periods ofdrying before they can be handled without danger of marring the surface. For this reason they present no advantagesover photographic materials. Finally, gelatin, when hardened by use. of formaldehyde or chromic salts, loses much of its: water sensitivity upon aging, making it diflicult to efiecta complete washout of unused chemicals.

It has now been found that a commercially satisfactory negative working diazo sulfonate foil can. be prepared by coating, on a transparent base, a layer of. a maleic anhydride-vinyl compound copolymer mixed with a com patible hydrophobic resin, treating the maleic anhydride copolymer to effect splitting, which may include partial arnidification of the-maleic anhydride linkages, and sensitizing the resulting water sensitive layer with a coating solution containing a diazo sulfonate and a coupling component. The material produced in this way contains a low-cost, easily processed, water sensitive layer which is dimensionally stable, permits ready washouto'f chemicals, is easily handled when wet, and dries very quickly after processing. Such' materials have unexpectedly superior properties from the standpoint of printing speed, opacity to actinic raysand especially improvedstability. In fact, it is possible for 'the firsttime by use of such foils to place the negative working diazo sulfonate process ona commercial scale, while permitting the selection of a printing system with such foilsto insure the obtainment of prints having an improved continuous or dot toneor a controlled contrast and gradation and lacking the grain found in photography.

Negative working diazo sulfonate foils containing a carrier layer rendered water sensitive as above and the processing of such foils by exposure under a pattern and developmentwith a base to produce a negative orpositive copy having, if desired, an improved continuous or dot tone or a very high contrast or controlled gradation constitute the purposes and objects of the present invention.

Our diazo sulfonate foil is realized byfirst dissolving a mixture of a maleic anhydride-vinyl compound copolymer anda compatible hydrophobic resin in an organic solvent to yield a solution suitable for coating. The proportions of copolymer to resin by weight may vary from at least 1:6 to 6:1 with excellent results; The lacquer solution is coated on a transparent base and dried down to form an essentially hydrophobic layer.

If reduction in curl is a desideratum the back side of the base is coated with an organic solvent, such as acetone, methyl Cellosolve, methyl Cellos'olve acetate, formic acid and the like, and again dried.

The film is next subjected briefly to the action of a water soluble base to effect hydrolysis of the maleic anhydride linkages of the copolymer. Dueto such hydrolysis, water sensitivity is imparted to the layer thus making it capable of taking up the sensitizing components from aqueous solution.

The sensitizingcomponents comprising the light sensitive diazo sulfonate, coupling component and otherusua'l adjuncts are now applied to-the water sensitivelayer from an aqueous solution. After drying, the material is ready foruse and may be exposed to actinic light under a negative photographic transparency and developed in ammonia vapors to produce a positive image. Conversely, by exposing under a positive, a negative image is obtained. Residual diazo sulfonate is removed from the unexposed areas and the backgrounds cleared by a brief washing in hot or cold water. Providing the sensitizing components have been selected for actini'c' o'p'acity', any

number of diazotype reproductions may be made from this intermediate foil with no significant darkening of the foil background.

Copolymers of maleic anhydride with vinyl compounds and their preparation arewell known in the art and, in this connection, reference may be made to Reissue Patent 23,514, granted June 24, 1952. Any of the copolymers of this patent may be employed in the preparation of our foils. Preferably, however, we use a copolymer of maleic anhydride with a vinyl ether, such as methyl vinyl ether, 2-methoxyethyl vinyl ether, 2-eth- 'oxyethyl vinyl ether, isobutyl vinyl ether and the like.

Similarly, while the proportions of the components of the copolymers may vary as indicated in the patent, we have found that best results ensue when utilizing equimolecular quantities of the monomeric components.

Any hydrophobic resin compatible with the copolymer may be employed and, in this connection, reference is made to the cellulose esters and mixed esters, such as cellulose acetate, cellulose propionate, cellulose acetate propionate, nitrocellulose and polyvinyl resins, such as polyvinyl acetate, polymethyl-methacrylate and the like.

The solvent employed for making the lacquer solution of copolymer'and resin may be any organic solvent capable of effecting dissolution of the components involved. Such solvents include acetone, butyl acetate, dioxane, tetrahydrofuran, ethyl methyl ketone, cyclohexanone and mixtures thereof with methyl Cellsolve and the like.

The quantity of solvent to copolymer and hydrophobic resin will vary depending upon the solvent used and the components to be dissolved therein. It may be stated, however, that the proportions must be such as to yield a solution of a viscosity suitable for coating. Manifestly, such viscosity may be determined by a simple test.

The hydrolysis of the copolymer is effected by the use of a water soluble base either in the vapor or liquid state. For instance, hydrolysis may be effected by a brief exposure of the film to ammonia fumes or, on the other hand, hydrolysis may be achieved by immersion in an alkaline solution of a water soluble base, such as ammonium hydroxide, or an alkylolamine, such as 2- aminoethanol, 2-dimethylaminoethanol and the like. Hydrolysis is allowed to proceed to such an extent that the layer becomes water sensitive and capable of taking up the sensitizing solution.

The sensitizing solution, essentially comprising the light sensitive diazo sulfonate and a coupling component, may be applied by immersion or by any other technique used in the diazotype art. Any light sensitive diazo sulfonate previously suggested for the negative working method may be employed, but we prefer to use such light sensitive diazo sulfonates as p-ethoxybenzenediazo sulfonate; p-methoxybenzenediazo sulfonate; 2,5-dimethoxybenzenediazo sulfonate and the like.

i The particular coupling components employed are not critical and those usually recommended for light sensitive diazotype materials have been found satisfactory. Examples of such couplers are resorcinol; phloroglucinol; ,tetrahydroxybiphenyl; dichlororesorcinol; dihydroxybiphenyl and l-(S-hydroxy-l-naphthyl)biguanide. Examples of other couplers that may be employed will be found in the Van der Grinten article referred to above.

The transparent base to which the copolymer hydrophobic resin is applied may be any of the usual transparent bases employed in the diazotype field. The only limitation on the base is that it be transparent and that it provide good wet and dry bonding between itself and the layer coated thereon. Preferably, we utilize a plastic material, such as cellulose acetate or higher acetylated celluloses.

It has been stated that the essential components of the sensitizing solution are the diazo sulfonates and the coupling components, but it is to be understood that the sensitizing solutions may contain the usual adjuncts em- 4 ployed in the manufacture of light sensitive diazotype materials. These include metal salts for intensification of the dyestufi image, such as ammonium sulfate, nickel sulfate, zinc chloride and the like; stabilizing agents such as thiourea, thiosinamine, sodium salt of o-sulfobenzaldehyde, naphthalenetrisulfonic acid and the like; acids acting to retard premature dye formation such as citric acid, boric acid, tartaric acid and the like; hygroscopic agents, such as glycol, glycerin and the like, and wetting agents, such as saponin, lauryl sodium sulfonate or the like.

The negative sulfonate process produces images of soft toe characteristics, and as a consequence it is possible to obtain some improvement in the appearance and gradation of the diazotype images produced by such process. However, it is often desirable when working with our diazo sulfonate foil to realize prints which have a good continuous tone or, conversely, prints having a very high contrast. We have ascertained, and this is an important feature of our invention, that improvement in the continuous tone properties may be obtained to a marked extent if the original negative silver image is copied onto our diazo sulfonate foil and the foil is then copied onto a positive working diazotype materialsuch as a diazo foil or print material. Prints made in this sequence, to wit, silver negative diazotype intermediate negative diazo sulfonate foil diazotype diapositivc or print produce distinctly better gradation in both the toe and highlight regions of the print. In this particular use, our diazo sulfonate foil is capable of yielding copies which have good continuous tone behavior in both highlight and shadow regions.

In the event that prints of very high contrast are desired, the above printing system may be modified so that the original silver negative is printed onto a positive working diazotype foil, the latter onto our intermediate diazo sulfonate foil, and said intermediate onto a positive working light sensitive material, such as a light sensitive diazotype positive or a light sensitive photographic positive. The final printing step, to wit, that involving the diazo sulfonate foil and the light sensitive positive material affords the great advantage of printing with the sensitized surfaces of the involved materials in contact. When proceeding in this fashion, we have found that the line contrast is excellent, although continuous tone is, of course, sacrificed.

The soft sensitometric toe typical of the prints obtained with negative diazo sulfonate foils has previously been commented upon. This characteristic may create ditficulties in certain applications. It is possible, however, to increase the low density contrast of our negative foils by placing a bleach-out layer in front of the diazo sulfonate image-forming layer, the function of which is to restrain the exposure in the high density region of the original. Any substance, evincing actinic bleach-out behavior and having spectral absorption in the same general region as the diazo sulfonate, is suitable, for example, the diazo sulfonate used in the image-forming layer or any other light sensitive diazonium compound, such as those derived from p-phenylenediamines. If a diazo sulfonate be employed, the base may be coated on both sides with the hydrophobic resin and copolymer, the coatings rendered water sensitive, and the image bearing layer impregnated with the diazo sulfonate and a coupling component, and the layer on the other side with the diazo sulfonate. The same procedure may be followed if a different bleachable diazo be used, such as a diazo from a p-phenylenediamine, i. e., the diazo from N-diethyl-p-phenylenediamine, N-methyl-N-butyl- -phenylenediamine and the like, in which case, such diazo is located in the layer opposite the image-forming layer. Conversely, the bleachable material may be located as a separate filter over the image-forming layer. In these modifications, exposure of the image-forming layers is effected through the bleachable layer.

The versatility of our diazo sulfonate foil is further 'ing examples, but it is to 'inal for 1 to 3 minutes,

enemas emphasized by the fact that it may be used to secure interposed between an original and a reproducing medium of moderately high contrast, serve to break up the continuous tone pattern of the original into a corresponding halftone dot pattern in the reproduction. Excellent results are secured when a contact screen is interposed between a continuous tone original and a .sheet of our negative foil, followed by exposure in .an Ozalid machine or a vacuum frame. Positive halftone copies of a continuous tone negative original result. .If desired, it is possible to combine such contact screens with the gradation controlling bleach-out layer previously discussed.

The invention will be further illustrated by the followbe understood that these are exemplary and not limitative.

Example I A cellulose acetate base is coated by a Bird film applicator with the following solution:

6 grams polyvinyl methyl ether-maleic anhydride copolymer 14 grams cellulose acetate (Hercules, grade L-Ll) 30 grams methyl Cellosolve 210 cc. acetone After drying, the layer is passed .over ammonia vapors several times, aired, and then sensitized from a Water solution of the following:

After exposure to ultraviolet through a negative origdepending upon the strength of the light source, followed by ammonia development, a sepia image is formed. After a 1 to 2 minute wash in *warm water, numerous reprints of good quality may be made with little or no degeneration of the intermediate.

Examplell A cellulose triacetyl base is coated with the following solution by means of areverse roll applicator:

2.5 grams polyvinyl isobutyl ether-maleic anhydride copolymer 2.5 grams polyvinyl acetate (medium viscosity) 35 cc. butyl acetate cc. methylCellosolve Hydrolysis of the layer was effected by immersion in a 3% dilute solution of ethanolarnine with subsequent doctoring of the excess solution. The sensitizingsolution is as follows:

.1 gram sulfosalicylic acid 4 grams zinc chloride 4 grams thiourea 0.1 gram isopropanol 100 cc. water After exposure and processing as in Example I, a deep -sepia image is obtained. Copies of excellent maximum density may be made from this'foil.

Example III The same procedure is followed as inExarnpileII, while using the following coating solution:

20 grams polyvinyl methyl ether-maleic anhydride polymer 20 grams cellulose acetate 342 cc. acetone 36 cc. methyl Cellosolve The sensitizing solution was that of Example H withca 3% solution of 1-(S-hydroxy-l-naphthyl)biguanide substituted for the resorcinol and phloroglucinol. Areddis'hbrown image of excellent opacity results.

Example IV A cellulose acetate base of cornbinedacetic acid :content of 60 to 60.9% is coated with the following organic .solvent solution:

70cc. acetone 10 cc. methyl Cellosolve 20 cc. denatured ethanol 3 grams-cellulose acetate combined acetic acid 55-56% 3 grams polyvinyl methyl ether-maleic anhydride copolymer The coated side of the base is then brought into contact with a 10% aqueous solution of 28 Baunr ammonia which reacts with the maleic anhydride effecting an opening and partial amidification of the :anhydride The excess of am- -rendering the coating hydrophilic.

passed through a monia is wiped off and the base is then drying channel to drive off the residual water and arnmonia.

'The hydrophilic coating is then overcoated with a sensitizing solution compounded as follows;

images. The foil is then washed with water to leach out the undecomposed diazo sulfonate thus fixing the image.

A positive image is obtained from a negative original and vice versa.

In the preceding examples, the molecular weight of the vinyl ether-maleic anhydride copolymer is normally such that a solution of 1 gram of thecopolymer in grams of 2-butanone has a viscosity of about 3 at'25 C. Higher and lower molecular weights, however, can be used successfully with due attention tosolvent balance, components of the solutionand the .coating method used.

The preparation of the water sensitive layers of the type contemplated for use herein as a carrier .for the sensitizing components is described in U. S..-Patent 2,7 5:6,- 163 granted July 24, 1956. It is to be understood that any of the techniques referred to in such application may be employed in our invention.

It -is to be equally emphasized that our invention is grounded on the discovery that the use of the water sensitive layers of the prior application as a carrier for diazo sulfonates permits for the first time the commercial development of a stable satisfactory negative working diazo sulfonate foil.

Example V A cellulose acetate base of combined acetic acid content of 60 to 60.9% is coated on both surfaces with the following organic solvent solution:

20 grams polyvinyl methyl ether-maleic anhydride copolymer 20 grams cellulose acetate 342 cc. acetone 36 cc. methyl Cellosolve Each coated surface of the base is then brought into contact with a 10% aqueous solution of 28 Baum ammonia to render each coated side hydrophilic. The excess of ammonia is wiped off and the base is dried to remove residual water and ammonia.

One surface of the base, which may be referred to as the image side, is then overcoated with the sensitizing solution of Example IV. The opposite surface, which may be termed the back side, is coated with an 8% solution of the sodium salt of p-ethoxybenzene diazo sulfonic acid.

The foil thus produced is processed by placing a transparent or translucent line original over the back side of the sensitized foil and exposing by means of a suitable ultraviolet source. The portions of the foil which are light struck are converted into the diazo which, when passed through a developing channel containing ammonia, couples with the tetrahydroxybiphenyl to produce a red-orange image. The foil is then washed with water to leach out the undecomposed diazo sulfonate to fix the image.

By exposing the double coated foil in the indicated manner the exposure in the high density region of the original is suppressed. In this way, the low density contrast of the foil is materially increased.

Example VI The negative working foil produced in accordance with Example IV is exposed to a continuous tone pattern between which and the foil is interpolated a contact screen prepared according to U. S. P. 2,311,071. By working up the foil as in Example IV, there is obtained a halftone dot pattern of the original.

Example VII.Preparatin of continuous tone prints A piece of high acetyl cellulose acetate filmbase is pretreated by dip coating with a lacquer of the following composition and dried:

4.5 grams polyvinyl methyl ether-maleic anhydride copolymer 4.5 grams cellulose acetate (56.5% acetyl, Hercules L-Ll) 100 cc. acetone 40 cc. ethyl alcohol 20 cc. methyl Cellosolve 200 cc. acetone In order to reduce curl, the back side of the pretreated filmbase is dip coated with a solution of the following composition and dried:

55 cc. acetone 40 cc. methanol cc. methyl Cellosolve acetate The surface is rendered Water sensitive by applying the following hydrolyzing solution by an applicator roll with a 15 second imbibition time and with the excess thereafter doctored off by means of a rubber bar:

5 cc. ethanolamine 0.1 gram saponin 100 cc. water The water sensitive surface next is sensitized with a sensitizing solution of the following composition:

A positive print is prepared by exposing the foregoing sensitized surface beneath a normal negative in a machine, sold by General Aniline & Film Corporation under the trademark Ozalid, at roughly 4 feet per minute, developed thoroughly with ammonia and fixed by washing in water for 1 to 3 minutes at a temperature of about 100 F. The surface can immediately be wiped dry by a cloth or cellulose towel and can be used immediately for producing further prints.

Thus, good black line continuous tone prints can be produced by reprinting the foregoing exposed and fixed foil onto a high brightness paper lacquered with a thin cellulose acetate dope, dried, and then sensitized with a solution of the following composition:

55 cc. water 25 cc. isopropanol 5 cc. *y-valerolactone 5 cc. n-butyl alcohol 5 cc. formic acid 2 grams anhydrous citric acid 2 grams thiourea 1 gram zinc chloride 5.2 grams resorcinol 1.6 grams 6,7-dihydroxynaphthalene-3sulfonic acid, so-

dium salt 0.4 grams 2,3-naphthalenediol 4 grams N,N-diethyl-4-amino-3-ethoxybenzene diazonium chloride, zinc chloride complex The resulting print material is exposed beneath the diazo intermediate continuous tone foil at a speed of about 6 feet per minute on an Ozalid machine and developed with ammonia. The resulting prints exhibit improved continuous tone behavior in both the highlight and shadow regions.

Instead of sensitizing a cellulose acetate coated print material with the last formulation given, it is possible to sensitize a transparent foil with substantially this same formulation. Diapositives made on this material, using the intermediate diazo sulfonate foil, likewise exhibit smooth continuous tone behavior in the highlight and shadow regions.

It should be noted that the materials cited must be used in the order given. For instance, if instead of the order-silver negative diazo sulfonate intermediate positivepositive working diazo materialthe order of the steps is changed to silver negativepositive working diazo materialdiazo sulfonate foil, it will be found in the latter case that the contrast is excessive in the region of the middle tones and the overall continuous tone properties are poor.

Example VIM-Preparation of prints with high contrast The procedure is similar to that of Example IV, excepting that the original negative is printed onto a high contrast positive working diazotype foil which, in turn, is printed onto the negative working diazo sulfonate foil coupler. It is to be emphasized, however, that any other high contrast positive working diazotype foil may be used.

By preceedin'g as in Example IV, the diazo sulfonate foil may be printed onto either a positive light sensitive diazotype material, such as that indicated above, or upon a positive photographic film with the sensitive surfaces of the foil and positive material in contact. The prints resulting from this procedure are notable for their exceedingly high contrast.

It is believed to be evident that in lieu of the light sensitive positive working diazotype material on which the negative working foil is printed as described above, any other positive working light sensitive diazotype film or paper may be used with equally good results. Examples of such materials, for instance, are given in U. S. application, Serial No. 350,528 filed by Peter T. Woitach, Jr., on April 22, 1953, now abandoned, and U. S. Patent No. 2,709,655 granted May 31, 1955.

Various modifications of the invention will be apparent to those skilled in the art and we, therefore, do not intend to be limited in the patent granted except as necessitated by the appended claims.

We claim:

1. A negative working diazo sulfonate material comprising a transparent base carrying a layer of a mixture of a copolymer of maleic anhydride with a vinyl compound and a hydrophobic resin compatible with said copolymer, said copolymer having been surface hydrolyzed with a moist water soluble nitrogenous base to render it hydrophilic and a light sensitive diazo sulfonate in which the sulf-onate group is directly attached to a nitrogen atom of the azo group and a coupling component imbibed to said layer. 7

2. The product as defined in claim 1, wherein the vinyl compound is a vinyl ether.

3. The product as defined in claim 1, wherein the vinyl compound is a vinyl aliphatic ether.

4. The product as defined in claim 1, wherein the ratio 10 of copolymer to hydrophobic resin by weight is from 1:6 to 6:1.

5. The product as defined in claim 1, wherein the hydrophobic resin is a cellulose ester.

6. The product as defined in claim 1, wherein the vinyl compound is a vinyl ether and the molecular weight of the copolymer is'such that a solution of 1 gram thereof in grams of 2-but'anone has a viscosity of about 3 at 25 C.

7. The product as defined in claim 1, wherein the diazo sulfonate is p-ethoxybenzene diazo sulfonate, and the coupling component is a polyhydric phenol.

8. A negative working diazo sulfonate material comprising a transparent base carrying a layer of a mixture of a copolymer of maleic anhydride with a vinyl compound and a hydrophobic resin compatible with said copolymer, said copolymer having been surface hydrolyzed with a moist water soluble nitrogenous base to render it hydrophilic, a light sensitive diazo sulfonate in which the sulfonate group is directly attached to a nitrogen atom of the azo group and a coupling component imbibed to said layer and a filter layer containing a compound bleachable by actinic light through which the sensitized surface is exposed.

9. A negative working diazo sulfonate material comprising a transparent base, each surface of which carries a layer of a mixture of a copolymer of maleic anhydride with a vinyl compound, and a hydrophobic resin compatible with said copolymer, said copolymer having been surface hydrolyzed with a moist water soluble nitrogenous base to render it hydrophilic, a light sensitive diazo sulfonate in which the sulfonate group is directly attached to a nitrogen atom of the azo group and a coupling component imbibed to the layer of one surface and a diazo compound bleachable by actinic light imbibed to said other surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,156,069 Schlack Apr. 25, 1939 2,197,456 Von Poser et a1 Apr. 16, 1940 2,699,392 Herrick et al. Jan. 11, 1955 FOREIGN PATENTS 402,7 37 Great Britain Mar. 5, 1942 

1. A NEGATIVE WORKING DIAZO SULFONATE MATRIAL COMPRISING A TRANSPARENT BASE CARRYING A LAYER OF A MIXTURE OF A COPOLYMER OF MALEIC ANHYDRIDE WITH A VINYL COMPOUND AND A HYDROPHOBIC RESIN COMPATIBLE WITH SAID COPOLYMER, SAID COPOLYMER HAVING BEEN SURFACE HYDROLYZED WITH A MOIST WATER SOLUBLE NITROGENOUS BASE TO RENDER IT HYDROPHILIC AND A LIGHT SENSITIVE DIAZO SULFONATE IN WHICH THE SULFONATE GROUP IS DIRECTLY ATTACHED TO A NITROGEN ATOM OF THE AZO GROUP AND A COUPLING COMPONENT IMBIBED TO SAID LAYER. 